It is well known to provide a vehicle with a four wheel drive apparatus to permit power transfer from all four wheels of the vehicle to the support surface at each wheel location. For example, all-terrain vehicles, pick-up trucks and the like may have a four wheel drive option to thereby maximize traction with the surrounding support surface.
Certain utility vehicles are also known to transmit power at each wheel location. Utility vehicles may include utility attachments that are useful for handling and/or moving various bulk materials, particulate, or the like. For instance, a utility vehicle, such as a fork lift, may be provided with a fork attachment to move bulk material stored in boxes. In another example, a utility vehicle with a shovel attachment may be used to transport particulate, such as soil, from one location to another. A conventional skid loader has various optional utility attachments for performing these various functions. Depending on the function desired, a fork, shovel, basket, or other utility attachment can be removably connected to an adapter plate attached to the lift arms of a utility vehicle.
Utility vehicles, such as a skid loader, are also known to incorporate a hydraulic system for powering the drive system, utility attachments and various other components of the vehicle. One conventional hydraulic apparatus 10 is illustrated in FIG. 1. The hydraulic apparatus 10 uses a pump 12, such as a fixed displacement hydraulic gear pump, to power the various components of the system. A fixed displacement pump will provide a constant hydraulic fluid flow rate determined by the rotational speed of the motor driving the pump 12.
As illustrated in FIG. 1, a valve bank 14 includes a first valve section 22 for controlling hydraulic fluid flow to a right side drive 34. The valve bank 14 further includes a second valve section 24 for controlling hydraulic fluid flow to a left side drive 36. The right side drive 34 includes a single drive motor 38 connected to a front right wheel of a vehicle and operable to drive the front right wheel. Similarly, the left side drive 36 includes a single drive motor 40 connected to a front left wheel of a vehicle and operable to drive the front left wheel.
Typically, each of the right and left side drives includes a chain to mechanically couple the front wheel to the rear wheel of the corresponding sides such that the front and rear right side wheel rotate at approximately equal angular velocities while the front and rear left side wheels also rotate at approximately equal angular velocities. Placing the right and left side drives in parallel in the hydraulic circuit provides the advantage of allowing an even distribution of work between the sides and permits the left and right sides to work independently and therefore operate efficiently despite differences in work load requirements between the sides.
However, the use of a chain for a drive link between the front and rear wheels may be undesirable in certain applications. Installation can be difficult and might require increased assembly time as chain tensioners and other tools must be used to optimize the installation of the mechanical mechanism. In addition, excessive maintenance costs can be experienced with a chain drive system as cleaning is typically required to remove foreign debris deposited on the chain that would otherwise cause undue wear and/or increase the power requirements to overcome excessive friction forces. The chain is also typically mounted to the front axle between the front motor and the front wheel, requiring the wheel to be offset a significant distance from the motor. This significant offset distance increases the bending moment resultant force within the bearing assembly of the motor, further increasing stresses and thereby decreasing the life of the motor. The increased space requirements to accommodate the chain drive mechanism further limit the lateral space available for the hydraulic fluid reservoir. Accordingly, the fluid reservoir might require an increased height, thereby undesirably raising the vehicle's center of gravity. Chain drives also frequently cause the vehicle to lurch forward when traversing a support surface and can result in jarring movements as the vehicle turns to navigate around obstacles. Jarring and lurching can occur as the chain transmits torque from the front wheel to the rear wheel due to tolerances and tension in sections of the drive chain. However, smooth precise movement is desirable and is often required for various applications such as when handling certain materials.